This invention relates to in situ measurements of earth formations traversed by a well borehole. In particular the invention relates to the measurement of the thermal neutron lifetime or thermal neutron decay time and the porosity of earth formations in the vicinity of a well bore.
The techniques used in the present invention include the generation as a function of time of a phase coherent intensity modulated cloud of fast neutrons in a well bore which results in the creation of a phase coherent thermal neutron cloud being produced as the fast neutrons are slowed to thermal energy by the materials in the vicinity of the well bore. Measurements of the relative phase of the thermal neutron cloud population density with respect to the fast neutron cloud generated at each of dual spaced detectors lead to the determination of parameters relating to the thermal neutron decay time or neutron lifetime of the formations in the vicinity of the well bore and the borehole fluid itself, together with the formation porosity.